Plastic injection molding is a well established method for producing a wide variety of plastic parts. A melt charge is built in the plasticizing section, then injected under high pressure into a closed mold held in a clamping station which must resist the pressure. After a suitable cooling time, the mold is opened to eject the finished part. To reduce the pressure created in the mold, a series of hot runner, sequential valves were developed to allow the melt to flow over shorter distances within the mold cavity to create a complete part. As cavity pressure build-up is related to the melt flow distance during fill, reducing this flow length with multiple valve gates, reduced the cavity pressure. The necessary software upgrade and valve sequence controls have become available for this lower cavity pressure injection molding process.
In order to further reduce the cavity pressure, various processes were developed where molten plastic was deposited into an open mold of a vertical acting clamping press. In one technique, molten plastic was injected through a series of valve gates into the bottom, stationary platen side of a down acting clamping station to form a series of puddles. The press closes on the still fluid material to fill out the mold cavity. Another version prepared a melt charge in the injection side of molding machine. When a sufficient molten charge was prepared, the entire injection unit moved forward into a open mold, attached to a vertical clamp press. As the injection section retracted out of the mold, the melt charge was deposited into the open mold in a controlled manner. This deposition of molten polymer into an open mold helped to reduce the cavity pressure based on third power affect of the cavity wall thickness on pressure generation during a typical closed mold filling process. However, like standard injection molding machines, these lower cavity pressure machines were designed to prepare and deposit a molten charge of plastic into a mold using a similar integrated, cycle driven process. Their main advantage is the ability to produce larger parts at lower clamping pressures or to capture decorative surface materials that would not resist the higher cavity pressures of a standard injection molding machine.
In reference to glass fiber reinforced thermoplastics processing, one method, described in U.S. Pat. No. 5,185,117, issued Feb. 9, 1993, utilized a vertical press and a two stage extrusion process where long glass fibers were introduced downstream into the plastic. The molten composite was discharged into an integrally attached accumulator in the form of a continuous log. As needed, a set length of molten plastic was cut from the log and loaded into the mold of a vertical acting clamping press, using a suitable transfer device such as a robot.
As described in U.S. Pat. No. 5,798,128, issued Aug. 25, 1998, another glass fiber reinforced thermoplastic process involved melting a plastic in a twin screw extruder, then introducing glass fibers into the melt stream within the extruder which exited into an accumulation chamber attached to the end of the extruder. A second syringe shaped accumulator is aligned with and connected to the first accumulator. A shutter opened and molten plastic was pushed into the second accumulator. At a predetermined fill level, the shutter closed and the syringe shaped accumulator was moved by a robot to either an open vertical press or connected to the fill port of a closed mold. Melt was pushed out of the same port opening used to fill the accumulator.
It is a feature of this invention to be able to produce polymer composites whereby collection devices carry polymer from a central plasticizing supply to clamping stations where the polymer can be directly deposited in the mold or transported via an intermediate carrier into the mold. This low pressure molding process allows inserts to be incorporated on the surface, as a core material or combined to form a finished product.
It is a further feature of the invention that the collection devices can transfer polymer into another chamber designed to accept and mix continuous fibers, chopped fibers or various combinations of chopped and continuous length fibers with the polymer using a continuous or discontinuous process. Surface materials and/or cores can be incorporated in the polymer composite.
Still another feature of the invention which allows the carrier deposited polymer or polymer composite to be shaped over the mold using compression dies that capture and form the polymer to produce a desired profile before placement in a mold.